As an object of establishing a LAN (Local Area Network) of an airbag system in an automobile, LAN protocols established by Safe-by-Wire Plus consortium and ISO (International Organization for Standardization)/IEC (International Electrotechnical Commission) are known (see Non-Patent Document 1 and Non-Patent Document 2).
The above-described standards define common specifications for an airbag control unit, a communication protocol between controllers which controls a peripheral device such as an airbag and a seatbelt, and a driver unit which drives a bus, and the requisite minimum specifications are determined.
And, the airbag control unit serving as a controller which controls the bus is called a “master”, and a side of controlling the peripheral device such as an airbag and a seatbelt is called a “slave”. The bus connects two lines of the master and the slave and becomes a path for providing the power supply and an operation of a bidirectional communication from a bus interface of the master side to a bus interface of the slave.
In the airbag control unit, a floating switch unit is used in the bus interface so that communication can be continued even when one side of the buses is short-circuited to a battery (power supply voltage) or a ground potential (reference potential).
For example, when the bus A is short-circuited to the battery, the voltage level of the bus B is shifted to the battery voltage side, and when the bus B is short-circuited to the ground potential, the voltage level of the bus A is shifted to the ground potential side, whereby the configuration is operated so that a voltage differential between the buses (the bus A−the bus B) is always maintained.
And, the floating switch unit comprises, for example, a switch which charges a first electrostatic capacitance from a battery power supply, a switch which provides the power supply from the first electrostatic capacitance to a transmitter circuit and the like, a switch which provides the power supply from the first electrostatic capacitance to the bus, a switch which charges a second electrostatic capacitance from the battery power supply, a switch which provides the power supply from the second electrostatic capacitance to a bus driver, and others.    Non-Patent Document 1: Automotive Safety Restraints BUS (ASRB) 2.0    Non-Patent Document 2: ISO/IEC 22896: Road Vehicles-Deployment and sensor bus for passenger safety systems